Highly efficient at converting sunlight to electricity, perovskite solar cells have
emerged as a revolutionary new technology with the potential to be more easily manufactured
and at a lower cost than silicon solar cells. Ongoing research, including at NREL,
focuses on moving perovskites beyond a laboratory setting.

The researchers took a close look at two organic-inorganic hybrid perovskite thin
films made of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3). Perovskite solar cells possess a polycrystalline structure with individual crystals
grains. These grains are adjacent to other crystals and the area where the crystals
touch is known as a grain boundary.

“The general assumption is that degradation starts with grain boundaries,” said Kai
Zhu, a senior scientist in NREL’s Chemistry & Nanoscience Department and co-author
of the paper. “We were able to show that degradation is not really starting from the
visible boundaries between grains. It’s coming from the grain surface.” As a result,
this implies that the surface of a perovskite solar cell should be targeted for improving
device performance.

The two thin films examined varied slightly. The first, with smaller grains, had a
power conversion efficiency (PCE) of 15 percent. The second, with larger grains, had
a PCE of 18 percent. Each film was protected by a layer of the plastic polymethyl
methacrylate (PMMA); earlier research showed unprotected films tended to degrade within
several hours under ambient conditions. The solar cells, illuminated by a focused
laser beam from below, were examined by a novel instrument, termed light-stimulated
microwave impedance microscopy (MIM). This allowed researchers to map the nanoscale
photoconductivity of the samples.

“With the MIM technique, for the first time we were able to visualize the intrinsic
nanoscale photo-response, which is of fundamental importance to solar cell performance,”
said Keji Lai, an assistant professor of physics at the University of Texas at Austin,
“Grain boundaries are usually the weak links in functional materials.” Lai worked
with his colleague, associate professor Xiaoqin Li, graduate student Zhaodong Chu,
and postdoc researcher Di Wu.

The analysis showed the photoconductivity of the 18 percent sample, which contained
a better crystallinity, was five to six times higher than that of the other thin film.
The perovskite thin films were tested over the course of a week in an area that was
74 degrees Fahrenheit and had 35 percent relative humidity. Little change in photoconductivity
was observed the first few days, but by the third day the measure began to drop as
water molecules moved through the PMMA coating. The drop in the photoconductivity
emerged from the disintegration of the grains and not from the grain boundaries, the
research found. In this instance, the scientists noted, the grain boundaries “are
relatively benign” and determined perovskite films with better crystallinity should
be a direction of future research for improving perovskite solar cell performance
and durability.

The research at University of Texas at Austin was funded by the National Science Foundation
and the Welch Foundation. The research at NREL was funded by U.S. Department of Energy
Solar Energy Technologies Office.

Other co-authors from NREL were Mengjin Yang and Philip Schulz.

NREL is the U.S. Department of Energy's primary national laboratory for renewable
energy and energy efficiency research and development. NREL is operated for the Energy
Department by The Alliance for Sustainable Energy, LLC.